Implements a single abstract method trait/class T using passed function or expression as implementation
of the sole abstract method.

Implements a single abstract method trait/class T using passed function or expression as implementation
of the sole abstract method. The argument passed may be either a function that must match the signature
of the abstract method or - in case the method does not take any arguments - an expression which will be returned
in the implementation of abstract method (as if the expression was passed as by-name parameter).